Electroacoustic transducer



g 1950 F. MASSA ELECTROACOUSTIC TRANSDUCER Filed June 8, 1945 Patented Aug. 15, 1950 Frank Massa, Cleveland Heights,"hi0' Application June 8, 1945, Serial No..598,27-0

'1 Claims. (o1. 17a-101) .1 :My invention is concerned with electro-acoustic transducers, and, more particularly with an improved design of such transducers sothat a low-cost precision assembly is possible is a compact space.

"It has'been the general practice, up to this time, to assemble head phones or microphones of the-diaphragm type by mounting the magnetic assembly or the 'piczo-electric crystal inside a housing and then to assemble the diaphragm to the housing. During this final diaphragm assembly procedure, the operating mechanism is shut off from view and there is no way to visually inspect the complete assembly for possible defects. Another disadvantage of this type of assembly which is particularly bad in the case of'hearing aid receivers is that the mounting of thevibrating structure to the rear wall of the outer housing causes sound to be radiated through-the housing to a relatively high'degree which results in disagreeable acoustic feedback through the system if the user tries to employ high gain in his set.

In'my improved design, I provide'for physical inspection of the completedmechanism and I also eliminate the direct mechanical connection between the vibrating element and the outer housing 'of the transducer. To accomplish this, I mount thediaphragm and the generating element on opposite sides ofa commonirame whichin the simplest form is a flat sheet of the desired shapethrough which a hole isprovided. By'mountin'g the diaphragm over one side of the hole and the vibratingmechanism on the opposite. side of the sheet,.a complete, electroacoustic assembly of wafer-like dimensions is made available and it is completely visible for mechanical inspection. This basic construction also permits improvement in the efficiencyof tiny hearing aid phonesof the crystal type. v

"For a crystal phone, the. use ofa square twister type ,of element, such-as iscommonly employed in crystal microphones .andhead phones, necessitates, in the conventional method of assembly, a. housing, of large diameter because the diagonalof the square .crystal must be approximately equal toltheradius of the housing. If a practical size of crystal. isto be employedinthis type of.

construction, an. undesirably large receiver for hearing aid, applications will result. .I 'o circumvent this undesirable size condition, various means have been employed. For example,1H. G, ModineiniPatent No. 2,284,063" employs aspecial housing in which one: corner of the crystalnests ina pocket which is-molded :as an" extension on a circular housing. .8. L. Lybarger,in Patent No. .2,3.67,7-26, allows the crystal drive corner :to

lie eccentrically inthehousing andhe uses an Another object ofmy invention is to decrease I the cost of manufacture of a precision microphone or sound: generator. i 1

A. further object of my invention is'to reduce the back radiationfrom a hearing aid receiver. vStill another object of my invention is to improve the efiiciency of small hearing aid .re-

ceivers. I

Another object of my invention is to permit the use of a relatively large vibrating mechanism inside a relatively small transducer, thereby improving the power handling capacity and sensitivity of the transducer. 1 A further object of my invention is to produce a rugged structure that will permitzthe economical use .of closetolerances'in itsm'anufacture Anotherobject'of; my invention is to reduce the cost ofsa. magnetic hearing aid receiver by eliminating the permanent magnet and using the plate current in the associated amplifier forp'olarizing. 4

Still another object of my invention is to com.- bine the diaphragm and supporting structure of a transducer into a common element and then.

which follow. "I'henovel features that I con sidericharacteristic of, my invention are set forth with particularity in the appended claims. The invention; itself,'however, both as to its organization'zand'method -oifope'ration, as Well as advan-.-

tages :thereof, will best'be understoodfrom the following description of several-embodiments thereof, when read in connection with the ac-q companying-rdrawings, .in'which-- Fig.1.is the front view offone'form of aihe'aring aid receiver which'employs the-iteachingsiof my invention.

Fig.2 is a section taken through the line 2-1. in Fig. l and shows'the internal construction of the receiver. I r Fig. 3 shows the rear view' of the hearing aid receiver of Figs. '1 and 2' with aportionc of the, rear housing cut away to expose the internal" Fig. 4 shows the rear view of a transducer assembly employing a square twister type of piezoelectric crystal coupled to a circular diaphragm.

Fig. 5 is a cross-sectional view taken along the line 5-5 of Fig. 4 and reveals the extrem compactness of the assembly.

Fig. 6 shows the rear view of another transducer similar to the one of Fig. 4 except that one corner of the crystal is chamfered and the mounting frame is circular instead of oval as in the previous figures.

Fig. 7 illustrates the use of the simpified ase sembly technique for a larger size unit such as a conventionalhead phone.

Fig. 8 is a cross-sectional view taken through line 88 of Fig. '7.

Fig. 9 shows a schematic circuit diagram for connecting the transducer of Fig. 2 to operate as a sound generator.

' Referring more particularly to Fig. 1, which shows the front view of a hearing aid receiver assembly, the reference character I indicates the front cover which is better shown in cross-section in Fig. 2, and reference character 2 represents the overall clamping ring which serves to keep the assembly together. The constructional details of the receiver are better shown in Fig. 2. The basic feature of the assembly is the use of a simple fiatplate 3 as the mounting structure. To'one side of the structure 3 is rigidly fastened the spool 1 which has been previously wound with the-coil of wire 8. The screws I l (shown in Fig. 3) hold the spool l to the frame't and the screws H hold the reed 6 clamped to the offset portion on the projecting end I of the spool l. The projecting surface I is so dimensioned that the reed 6 lies along the center line of the spool I. The reed 6 is a bi-metallic magneto-striction assembly' as described in my co-pending application 2388;691, filed April 16, 1945, now Patent No. 2,475,148, granted July 5, 1949, and when current flows through coil 8, a bending of the reed results in the top or unfastened end of the reed 6 transmitting a vibration through the connecting rod 5 to the diaphragm 4. In order for the frequency of vibration of the reed 6 to correspond to the alternations of the current in coil 8, it is necessary that an initial D.-C. polarizing flux be established through the reed 6. One method of establishing the D.-C. flux 'will be described in connection with Fig. 9 and another method will be described in connection with Figs. 10 and 11. The electrical connections to the coil 8 are made through a pair of spring contacts 9 (shown in Figs. 2 and 3). The spring contacts 9 are attached to the supporting frame 3, as indicated in Fig. 3, and the coil leads l5 are electrically connected to each of'the terminal springs 9. On the rear of the housing In is a projection I2 into which the free tips of the terminal springs 9 enter, as indicated in the cross-sectional view Fig; 2." Two holes are drilled into the projection l2 parallel with the axis of the reed v6 with each hole directly over one of the free ends of the springs 9. This is'to permit the insertion of a pair of pins on the end of a cord, not shown; for establishing electrical connection to the unit.

After the spool I and the diaphragm 4 are rigidly attached to the frame structure 3with the drive pin 5 accurately aligned through the center of the diaphragm 4, a drop of cement I3 is placed over the exposed tip' of the drive pin 5, completing the assembly of the transducer element. The entire transducerasse'mbly consisting of the diaphragm 4, the mounting structure 3, the spool l, the reed 6, and the drive pin 5 is now a simple, compact unit which permits complete visual examination of the working parts before further assembly. The particular structure indicated in Fig. 2 represents an insert type of hearing aid receiver as generally employed with hearing aid equipment. In order to improve the efiiciency of such a small receiver, I reverse the conventional position of the diaphragm and have the apex facing outward from the inner chamber, as shown in Fig. 2. By arranging the diaphragm as shown, I permit an increased volume of air to remain behind the diaphragm and, at the same time, de-

, structure crease the volume of air between the diaphragm and the ear. By increasing the volume of air behind the diaphragm, there will be a reduced stiffness tending to suppress the vibrations of the diaphragm and, therefore, I am able to produce larger sound pressures in the ear canal for a given electrical input to the driving system. The complete receiver is assembled by placing the front portion 1 and the rear housing [0 over opposite sides of the frame structure 3 and a pre-formed, thin metal band 2 is rolled over, as indicated, for holding the assembly together. an oval-shaped structure in order to accommodate a longer reed 6 than could have been used if the structure were kept round with a diameter equal to the diameter of the diaphragm as shown. It is possible to make the housing ID, the frame 3, the diaphragm 6, and the cover plate I of a plastic such as polystyrene and assemble the parts, as shown, by first moistening the contact areas with a solvent for the plastic such as acetone, thus causing the parts to be effectively welded together and the rim 2 may be omitted. To the free end of the reed 6 is attached a drive pin 5 which projects through the center of the hole in frame 3. The diaphragm 4 is cemented to the frame 3 and a hole through its apex is allowed to pass over the exposed end of the drive pin 5.

Fig. 4 shows a rear view of a transducer assembly similar to that of Fig. 3 except that a piezoelectric crystal IQ is employed as the transducer 7 element. Fig. 5 is a cross-sectional view taken along the line 5-5 of Fig. 4. Referring simultaneously to Figs. 4 and 5, the reference character [6 represents the main supporting structure which has a pair of projections l 7 extending into a circular opening, as indicated. The surfaces of the projections H are slightly under-cut on the diaphragm side of the frame l6, as shown in Fig. 5, in order to provide sufiicient clearance for the corrugation near the outer edge of the diaphragm 4. The projections I! merely serve to furnish increased area for attaching the terminal springs 18 and the corners of the crystal l9. Basically, the assemblyof the transducer of Figs. 4 and 5 follows the same method described in connection with Fig. 2. There is employed a square piezoelectric crystal l9, consisting of two shear plates 20 and 2| cemented together such that when three corners of the crystal are mounted, the fourth corner will vibrate corresponding to the alternating potential applied to the crystal faces.

Such crystals are well known and are commonly employed in piezo-electric microphones and receivers of conventional design. The crystal I9 is attached to the plane surface of the mounting I 6 by means of three small pads 22. The crystal is located so that the drive pin 23 passes accurately through the center of the opening in the frame H5. The diaphragm 4 is cemented or otherwise aflixed to the oppositeside of the frame l6, as indicated in Fig. 5, and a drop of eement' l3 applied over the exposed end'of the I have employed drive pin 23 securing it to the diaphragm 4. A pair of spring terminals 18 are arranged to serve the same purpose described in connection with the spring terminals 9 in Fig. 2. The transducer assembly of Figs. 4 and 5 possesses the advantage of being extremely simple and is capable of precision assembly without requiring costly methods. The assembly is also available for complete visual inspection before inserting it into its housmg.

In Fig. 6 is shown the rear view of a modified form of the structure shown in Fig. 4. In Fig. 6, the mounting structure 24 replaces mounting structure 16 of Fig. 4 and results in a circular assembly instead of the oval assembly previously indicated. The crystal 25 has its lower corner chamfered, as shown, in order to minimize the outside diameter of the structure 24. A crosssectional view of Fig. 6 has not been shown since it would be analogous to the view shown in Fig.

5. The only difference in connection with Fig.

6 is that the auxiliary projections II, as required in Fig. 5, are not needed in the construction of Fig. 6.

In Fig. '7 is shown the rear view of another form of my invention which is particularly adaptable for a structure permitting a relatively large diaphragm such as, for example, a conventional head phone as generally worn over the head with a supporting head band. Fig. 8 is a cross-sectional view taken along the line 88 of Fig. 7 Referring simultaneously to Figs. '7 and 8, the reference character 26 indicates the basic frame structure to which is attached the crystal l9 by means of the small pads 22. A pair of terminal pins 21 are assembled into the projecting areas 28 of the frame structure 26 and serve to make contact to a pair of springs mounted inside the rear housing into which the transducer assembly will be placed. The drive pin 23 is attached to the diaphragm 4 by means of the cement [3. The complete assembly of the transducer element of Fig. 8 is very similar to the assembly described in Fig. 5 except for the fact that a circular supporting structure 26 replaces the oval-shaped structure i6 of Figs. 4 and. 5. In the construction of Figs. 7 and 8, the completed structure is circular in shape and the diaphragm occupies the full diameter of the unit. The reversed diaphragm arrangement previously described is also employed in Fig. 8 to permit higher efliciency by I having maximum volume of air behind the diaphragm when the structure is placed in its housing. In the structure of Fig. 7, the crystal l9 could be replaced by a larger crystal, as shown in Fi 6.

Fig. 9 shows the schematic connection of the receiver of Fig. 2 to the output circuit of an amplifier so that the polarizing flux is automatically supplied by the D.-C. plate current of the tube. The coil 8 is connected between the plate battery 13+ and plate of the tube 29, and the number of turns and size of wire on coil 8 are adjusted so that the proper required magnetomotive force is produced by the plate current taken by the tube. The A.-C. current that flows through the coil 8 when audio signals are impressed on the grid of the tube 29 will cause direct actuation of the reed 6 and its associated drive pin 5. By employing the connection just described, it is possibe to operate the transducer without the use of a permanent magnet and without the use of any coupling elements between the transducer and the vacuum tube. Although the circuit of 9 has been shown in connection with the magnetostriction element of Fig. 2, it is also possible to employ the same circuit connections to eliminate the use of a permanent magnet in conventional types of magnetic transducers employing a moving magnetic armature.

My invention has been described in connection with several embodiments which I have chosen to illustrate the basic ideas; however, it will be obvious to those skilled in the art that numerous deviations will be possible from the specific details shown, and I, therefore, desire that my invention shall not be limited except insofar as is made necessary by the prior art and by thespirit of the appended claims.

I claim as my invention:

1. In an electro-mechanical transducer, a supporting structure having a pair of parallel surfaces lying on opposite sides of said structure, a vibratile diaphragm mounted on said supporting structure and against one of said parallel surfaces, transducer means mounted on the supporting structure and against the opposite parallel surface thereof, means connecting said transducer means to said diaphragm, a housing structure, spring terminal means attached to said supporting structure and associated with said housing structure for providing electrical connection through said housing structure, and means for securing said housing structure to said supporting structure.

2. In a hearing aid receiver, a supporting structure having a pair of parallel surfaces lying on opposite sides of said structure, a vibratile diaphragm mounted on said supporting structure and against one of said parallel surfaces, transducer means mounted on the supporting structure and against the opposite parallel surface thereof, means connecting said transducer means to said diaphragm, a housing structure including as one of its sides an adapter plate, means for securing said housing structure to said supporting structure, and spring terminal means attached to said supporting structure and associated with said housing structure for providing electrical connection through said housing structure.

3. The invention set forth in claim 1, characterized in that said transducer means is an electromagnetic device.

4. The invention set forth in claim 1, characterized in that said transducer means comprises a piezo-electric crystal.

5. The invention set forth in claim 1 characterized in that said supporting structure is oval-shaped.

6. The invention set forth in claim 1 characterized in that said supporting structure is oval-shaped and said diaphragm is circular.

7. The invention set forth in claim lucharacterized in that said supporting structure is circular and said diaphragm is eccentrically mounted on said supporting structure.

FRANK MASSA.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,100,500 Woodruff et al Nov. 30, 1937 2,145,449 Lockhart Jan. 31, 1939 2,284,063 Modine May 26, 1942 2,345,996 Anderson et al. Apr. 4, 1944 2,391,924 Rutter et al. Jan. 1, 1946 

